This invention relates to probes for sensing the position of an object. It may be used on positioning apparatus such as machine tools, coordinate measuring machines, measuring robots and the like.
Particularly in numerically controlled machine tools, it is known to exchange a cutting tool for a position-sensing probe. The probe may for example be of the touch trigger type, such as shown in U.S. Pat. No. 4,153,998 (McMurtry), which is incorporated herein by reference. Such a probe has a workpiece-contacting stylus and issues a trigger signal when the stylus contacts a workpiece. Alternatively, the probe may be of the “analogue” or “measuring” or “proportional” type, which provides an output which is a measure of the deflection of the stylus relative to a body of the probe. Rather than having a workpiece-contacting stylus, any of these types of probe may instead sense the workpiece using optical, capacitive, inductive (e.g. using eddy currents) or other non-contact techniques. The present invention may be used with any of these and with other types of probes.
Since such a probe for use in machine tools is exchangeable with cutting tools, it can be difficult to provide wires or cables to connect the probe's output signal to the controller of the machine. Consequently, various wireless signal transmission techniques are used, including inductive transmission, optical transmission and radio transmission. The probe then needs to be battery powered. To conserve battery life, it is desirable that the probe should only be switched on when in use.
A known technique for switching on such a probe is described in U.S. Pat. No. 4,599,524 (McMurtry), which is incorporated herein by reference. Here, after the probe has been inserted in the spindle of a machine tool, its battery is switched on by a brief rotation of the spindle. A centrifugal switch within the probe responds to such rotation. After use, the battery may be disconnected by a further such rotation, or by a delay element within the circuit of the probe which times out after a predetermined period of non-use of the probe.
Probes using such centrifugal switches have been successful commercially, but the centrifugal switches used are fairly bulky. Consequently, if a probe having a small physical size is required, such centrifugal switches cannot be used, and it has been necessary to use other methods for switching the probe on. A further problem with such centrifugal switches is that they are relatively expensive.